1. Field of the Invention
This invention relates to a new process for the production of chlorofluorocarbon-free, rigid polyurethane foam moldings having a huih surface hardness and a gross density of at least 300 kg/m.sup.3 using water and/or carbon dioxide in combination with special zeolites as blowing agents.
2. Description of the Prior Art
The production of rigid polyurethane foam moldings is described, for example, in DE-AS 1,694,138. The moldings in question are moldings having a compact outer skin and a differential distribution of density over the cross-section of the molding. Foams such as these are generally known as "integral foams."
The use of low-boiling solvents as blowing agents is crucial to the formation of this "integral structure." Particularly suitable solvents are chlorofluorocarbons (CFC) R 11 and R 113.
The importance of these blowing agents to the integral structure is emphasized by the fact that foaming formulations, in which the CO.sub.2 produced by the reaction of H.sub.2 O with isocyanate is used solely or even only partly as blowing agent, are unable to form this particular foam structure because even small quantities of water produce a skin permeated by fine cells. The result of this is that the surface hardness (Shore A, D) of such foam moldings is considerably lower than that of comparable integral foams produced with the blowing agents R 11 or R 113.
In many practical applications, however, surface hardness plays a crucial role. For example, in the production of moldings for monitor housings or switch boxes, a surface hardness of at least 70 Shore D is absolutely essential because the moldings must be scratch-resistant. Previously, it has only been possible for a standard density of 600 kg/m.sup.3 to achieve this hardness with the usual formulations when low-boiling chlorofluorocarbons are used as blowing agents.
An object of the present invention is to provide a new process for the production of rigid polyurethane foam moldings having a compact surface skin of extreme hardness which does not depend on the use of chlorofluorocarbons as blowing agents.
Surprisingly, this object is achieved by the use of water and/or carbon dioxide in combination with selected zeolitic adsorbents as blowing agents and, at the same time, by the use of selected polyhydroxyl compounds as the principal reactant for the polyisocyanate component.
The production of polyurethane foams having a compact surface and a cellular core using alkali aluminum silicates of zeolite structure is already known from DE-OS 1,804,362. However, the foams according to this prior publication are light-density foams having a gross density of about 80 to120 kg/m.sup.3. The main concern of the authors of this prior publication was to produce non-shrinking foams despite the use of chlorofluorocarbons. Accordingly, these blowing agents are used in all of the examples although water is also mentioned as the sole blowing agent in Claim 1 of the prior publication. However, this cannot in any way suggest the process according to the invention which is described in detail hereinafter, because the object is the invention (extreme surface hardness despite not using chlorofluorocarbons) is not discussed in the prior publication nor is the proposed solution according to the invention, according to which water, preferably in combination with selected zeolites, is used for the production of polyurethane moldings of comparatively high density and high surface hardness.